splitting UV islands to have each face separated by small amount
Margin

In order to create a small gap between the UV islands you can use the Margin factor it is shown in the tool options panel while performing most of the UV mapping related operations (Unwrap, Smart Unwrap, Lightmap,...)  or even in the Pack Islands command.



This will create an uniform margin along the island's boundaries. That's probably the supposed way to achieve the result that fit best the baking-with-margins workflow.

Scale islands individually

If you are not interested in a uniform margin, but you just need to scale each face down, there is the possibility to change the scaling (S) pivot as shown in the image sequence below: